When assembling or dismantling pipe strings that are run into or down to a subterranean well, for example, for the recovery of oil it is usual to use so-called power tongs or torque tongs to connect or disconnect lengths of pipe from the pipe string. A common configuration is that an upper pipe section is manoeuvred and held by a gripping device, whilst a pipe section located below is rotated by means of a rotary torque tong. Another configuration is that the pipe string is held by a first power tong and/or by slips in the drill floor, whilst a rotary torque tong rotates a pipe member thereabove with the necessary torque. The rotary torque tong is equipped with gripping jaws (Norwegian gripebakker) for holding the pipe centrally in the tong so that when the torque tong rotates, the pipe member will also rotate about its longitudinal axis. It is an advantage if the rotational axis of the torque tong and the axial axis/longitudinal axis of the pipe member are positioned so as to be essentially coincidental.
In connection with the development of new torque tongs there has arisen a need to develop a pipe clamping device which ensures that the torque tong turns about a centre. It is also desirable that this pipe clamping device should be capable of being integrated as a part of the torque tong, although it can also be used separately from the torque tong. A person of skill in the art will also understand that the pipe clamping device will be capable of being used together with other types of equipment for connecting a pipe section to a pipe string than that explicitly mentioned here.
Pipe clamping devices according to the prior art often use hydraulically operated gripping devices that position and hold the pipe for engagement with the torque tong. It has proven to be difficult to obtain a precise arrangement where gripping devices are simultaneously brought to the tubular centre for centring the pipe in such a way that the torque tong is assured rotation about the tubular centre. A problem associated with the control of the gripping devices is that it is difficult to obtain good control of the positioning of hydraulically controlled gripping devices, without the use of sensors and monitoring systems. The use of sensors and monitoring systems results in a complex control system, and therefore there has arisen a need to produce a simple synchronising mechanism for the gripping devices that is based on mechanical principles.
Since the forces that act when the gripping devices grip about the pipe are extremely large, it is important that the pipe clamping device is provided such that damage is prevented in the event that not all the gripping devices are in contact when the system is actuated for the exertion of force of the gripping devices against the pipe wall. In the case where the pipe clamping device comprises three gripping devices, a situation may arise in which two of the gripping devices are brought into contact with the pipe, whilst the third device is not in contact with the pipe wall when the gripping device is activated to exert holding force against the pipe. In such a case, extremely large forces are at work in the system, and in the worst case this can result in breakdown of the pipe clamping device. There is therefore a need to adapt the pipe clamping device in such a way that the forces exerted by the gripping device that is not in contact with the pipe are taken up by the arrangement of the pipe clamping device. This is achieved in that the pipe clamping device is provided with a synchronisation system which is not self-destructive.
Furthermore, the pipe clamping device may be provided with a system to detect whether the gripping devices grip a pipe which is located outside a tolerance zone that is defined for the tubular centre of the pipe clamping device, in that the arrangement can be provided with switches that are capable of interrupting the sequence if the pipe lies outside the tolerance zone.
In addition to the invention having its natural use in oil-related operations, and in particular in connection with the joining of pipe lengths that are to be used in drilling-related operations, for example, drill pipes, drill collars and casings, the invention will also be useful in applications where it is necessary to obtain a positioning and holding of a pipe within a particular positioning area.